This invention is a circuit for correcting for polygon scan errors in a raster scanner, and more specifically to a circuit for modulating a clock frequency to match a scan linearity error profile to effect scan correction.
An optical scanner may consist of a rotating light deflector which displaces a spot of light across a flat “scan line”. “Scan deflectance” is a function which relates the position of the spot on the scan line to the angle of the deflector. Its angular derivative “scan revolvance”, relates the change in position of the spot on the scan line to the angle of the deflector. By varying the frequency of the pixel clock, the pixel clock is used to regulate the flow of data into or out of the scanner system.
For the scanning system to function properly, there must be a well defined relationship between the pixel clock frequency and the scan revolvance. For example, if a constant density of information on the scan line is required, then it may be appropriate to use a pixel clock of constant frequency and a constant scan revolvance. Generally, the uncorrected scanner's scan deflectance is non-linear. Therefore, its scan revolvance is not constant. For instance, a typical cause of non-linearity is that the distance of the spot on the scan line to the center of rotation of the deflector increases as the spot moves away from the center of the scan line. Other non-linearities may be caused by the particular design of the scanner system. Since the pixel clock used to regulate the flow of data into or out of the scanner system is generally constant during a scan, efforts have been made to make the scan revolvance constant also. Lenses may be employed to correct for the above-mentioned non-linearity by bending the spot toward the center of the scan line as the spot moves away from the center.
The speed at which the spot moves along the scan line is called “scan velocity”. Ideally, scan velocity is proportional to scan revolvance. However, there are several factors which affect the constant of proportionality.
First, a scanning system may employ a series of deflectors, called facets, in a polygonal arrangement to increase the efficiency of the scanner system. Due to manufacturing anomalies, all the facets of the polygon are not necessarily at the same optical distance from the scan line. This means that each facet has its own constant of proportionality relating scan revolvance to scan velocity. This is called “polygon signature”. Second, the deflector(s) may not turn at a constant rate. This is called “motor hunt”. These system scanning errors may be minimized by either improving the polygon and drive systems, or by varying the clock rate to compensate for the errors.